Tinnitus is an abnormal neurological behaviour arising from loss of signal through the ear. While the precise causes of tinnitus are not fully understood, certain analogies are employed to describe the likely causes. For example, it is thought that tinnitus is often caused by a physical hearing impediment such as damage to the hairs in the cochlea. In an attempt to compensate for the missing audio information the brain raises amplification and the gains in recurrent loops to such an extent that false signals are generated, similar in principle to the ringing noise that can occur when the volume of an audio amplifier in an auditorium is raised too high. Alternatively, one may imagine an electrical water pump whose water supply is suddenly restricted. The pump oscillates and vibrates in a desperate attempt to compensate for the loss of input. Tinnitus may be thought of as essentially arising from the same types of mechanism: loss of signal through the ear results in increased spontaneous and oscillatory activity in the associated neurons in the brain. This activity is perceived as illusory sound by the sufferer.
Tinnitus sufferers are significantly more likely to perceive an illusory audio after-effect known as the Zwicker Tone. The Zwicker Tone is induced by exposing the individual to broad-spectrum noise (20 Hz-20 KHz) with a spectral gap (silence) at an arbitrary frequency. When the noise is removed the individual perceives a ‘ringing’ at the frequency of the spectral gap. This suggests that in order to compensate for the unequal cochlear sensitivity across frequencies, the brain introduces frequency dependent sensitivity or gain similar to a ‘graphic equalizer’ on a stereo. At the frequencies that our cochlea is less sensitive, the brain increases the gain in that frequency band to compensate. In frequency bands where sensitivity falls below a minimum threshold, the brain increases the gain to pathological levels. This manifests as illusory noise, ringing or even chaotic oscillation, the most commonly described effects of Tinnitus.
A very large number of treatments have been proposed for tinnitus, including radiosurgery, direct stimulation of the auditory nerves, pharmacological treatments, psychological treatments, and treatment by playing external sounds to the patient. While many such treatments provide relief in some groups of patients, at present there is no reliable treatment for all patients, and the present invention aims to provide a further alternative approach.